Gracilaria vermiculophylla (Ohmi) Papenfuss, 1967 (Rhodophyta, Gracilariaceae) in northern Europe, with emphasis on Danish conditions, and what to expect in the future

Gracilaria vermiculophylla, a red macroalga from the West Pacific, was discovered in western Germany (the Wadden Sea) in 2002 and has since also been observed in Sweden (from about 70 km south to about 80 km north of Goteborg), Denmark (Wadden Sea, Horsens Fjord, Limfjorden, Vejle Fjord, Holckenhavn Fjord, Oster Hurup Harbor) and eastern Germany (Kiel Bay). Today, less than 5 years following its first observation in the Wadden Sea the invader is common in many invaded regions, often being amongst the most abundant macroalgal species. G. vermiculophylla is successful in shallow protected soft-bottom estuaries and bays, typically in association with ubiquitous native invertebrates (lugworms, tube-building worms, mussels, cockles, snails). The invertebrates provide substratum for holdfast attachment and thalli incorporation, most likely increasing the stability of local G. vermiculophylla populations. We hypothesize that this substratum provision is highly important for its general invasion success. We also confirm that G. vermiculophylla can maintain growth at all salinities experienced along Danish coastlines (8.5-34 psu). In addition, laboratory experiments indicate that the ubiquitous grazer Littorina littorea has the potential to control G. vermiculophylla growth under specific environmental conditions, but also that L. littorea may facilitate small-scale dispersal within invaded locations, because grazing increases thalli fragmentation rates. Given its widespread distribution, rapid range expansion, and known ecological traits, G. vermiculophylla is clearly a permanent resident of northern European waters.